Alcohols have a variety of industrial and scientific applications such as a beverage (i.e., ethanol), fuel, reagents, solvents, and antiseptics. For example, butanol is an important industrial chemical and drop-in fuel component with a variety of applications including use as a renewable fuel additive, a feedstock chemical in the plastics industry, and a food-grade extractant in the food and flavor industry. Accordingly, there is a high demand for alcohols such as butanol, as well as for efficient and environmentally-friendly production methods.
Production of alcohol utilizing fermentation by microorganisms is one such environmentally-friendly production method. However, in the production of butanol, for example, some microorganisms that produce butanol in high yields also have low butanol toxicity thresholds. Removal of butanol from the fermentation as it is being produced is a means to manage these low butanol toxicity thresholds. Thus, there is a continuing need to develop efficient methods and systems for producing butanol in high yields despite the low butanol toxicity thresholds of the butanol-producing microorganisms.
In situ product removal (ISPR) (also referred to as extractive fermentation) can be used to remove butanol or other fermentative alcohols from the fermentation as it is produced, thereby allowing the microorganism to produce butanol at high yields. One ISPR method for removing fermentative alcohol that has been described in the art is liquid-liquid extraction (see, e.g., U.S. Patent Application Publication No. 2009/0305370). In general, with regard to butanol fermentation, the fermentation broth which includes the microorganism is contacted with an extractant at a time before the butanol concentration reaches, for example, a toxic level. The butanol partitions into the extractant decreasing the concentration of butanol in the fermentation broth containing the microorganism, thereby limiting the exposure of the microorganism to the inhibitory butanol.
In order to be technically and economically viable, liquid-liquid extraction requires contact between the extractant and the fermentation broth for efficient mass transfer of the alcohol into the extractant; phase separation of the extractant from the fermentation broth (during and/or after fermentation); efficient recovery and recycle of the extractant; and minimal decrease of the partition coefficient of the extractant over long-term operation. Extractant can become contaminated over time with each recycle, for example, by the build-up of lipids present in the biomass used as feedstock for fermentation, and this contamination can lead to a concomitant reduction in the partition coefficient of the extractant.
In addition, the presence of undissolved solids during extractive fermentation can negatively affect the efficiency of alcohol production. For example, the presence of the undissolved solids may lower the mass transfer coefficient, impede phase separation, result in the accumulation of oil from the undissolved solids in the extractant leading to reduced extraction efficiency over time, slow the disengagement of extractant drops from the fermentation broth, result in a lower fermentation vessel volume efficiency, and increase the loss of extractant because it becomes trapped in the solids and ultimately removed as Dried Distillers' Grains with Solubles (DDGS).
Thus, there is a continuing need for alternative extractive fermentation processes that reduce the toxic effect of the fermentative alcohol such as butanol on the microorganism, and which can also reduce the degradation of the partition coefficient of an extractant. The present invention satisfies the above needs and provides processes and systems for the fermentative production of alcohols such as ethanol and butanol.